Access channel structure for wireless communication system

ABSTRACT

A technique for efficient implementation of pilot signals on a reverse link in a wireless communication system. An access channel is defined for the reverse link such that within each frame, or epoch, a portion is dedicated to sending only pilot symbols. Another portion of the frame is reserved for sending mostly data symbols; however, within this second portion of the frame, additional pilot symbols are interleaved among the data symbols. The pilot symbol or preamble portion of the access channel frame allows for efficient acquisition of the access signal at the base station, while providing a timing reference for determining the effects of multipath fading. In particular, a pilot correlation filter provides a phase estimate from the pilot symbols in the preamble portion, which is then used to decode the data symbols in the payload portion. An access acquisition portion of the receiver uses the phase estimates provided by the pilot correlation filter to process the output of a data symbol correlation filter. The additional pilot symbols embedded in the payload portion are used in a cross product operation to further resolve the effects of multipath fading.

RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisionalapplication Ser. No. 60/181,071, filed on Feb. 8, 2000, entitled “AccessChannel Structure For Wireless Communication System”. The entireteachings of the above application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to the field of wirelessdigital communications and more particularly to a technique for encodingaccess channel signals.

[0003] The increasing use of wireless telephones and personal computershas lead to a corresponding demand for advanced wireless communicationservices which were once thought only to be meant for use in specializedapplications. In particular, wireless voice communication first becamewidely available at low cost through the cellular telephone network. Thesame has also become true for distributed computer networks, whereby lowcost, high speed access to data networks is now available to the publicthrough Internet Service Providers (ISPs). As a result of the widespreadavailability of both technologies, the general population nowincreasingly wishes to be able to access the Internet using portablecomputers and Personal Digital Assistants (PDAs) over wireless links.

[0004] The most recent generation of wireless communication technologiesmakes use of digital modulation techniques in order to allow multipleusers to share access to the available frequency spectrum. Thesetechniques purportedly increase system capacity for a radio channel of agiven available radio bandwidth. The technique which has emerged as mostpopular within the United States is a type of Code Division MultipleAccess (CDMA). With CDMA, each transmitted radio signal is first encodedwith a pseudorandom (PN) code sequence at the transmitter. Each receiverincludes equipment that performs a PN decoding function. The propertiesof the PN codes are such that signals encoded with different codesequences or even with different code phases can be separated from oneanother at the receiver. The CDMA codes thus permit signals to betransmitted on the same frequency and at the same time. Because PN codesin and of themselves do not provide perfect separation of the channels,certain systems have added an additional layer of coding, and/or usemodified PN codes. These additional codes, referred to as orthogonalcodes, and/or modified PN codes encode the user signals so that they aremathematically exclusive in order to further reduce interference betweenchannels.

[0005] In order for the CDMA code properties to hold true at thereceiver, certain other design considerations must be taken intoaccount. One such consideration involves the signals traveling in areverse link direction, that is, from a field unit back to the centralbase station. In particular, the orthogonal properties of the codes aremathematically optimized for a situation where individual signals arriveat the receiver with approximately the same power level. If they do not,interference between the individual signals which arrive at the basestation increases. Precise control over the level of each signaltransmitted on the reverse link is thus critical.

[0006] More particularly, most CDMA systems are structured such that theforward link channels, that is, the channels carrying information fromthe base station towards the field unit, are different from the reversechannels. The forward link typically consists of three types of logicalchannels known as the pilot, paging, and traffic channels. The pilotchannel provides the field unit with timing and phase referenceinformation. Specifically, the pilot channel contains a sequence of databits that permits the field unit to synchronize its PN decoding functionwith the PN coding used in the base station. The pilot channel is,therefore, typically transmitted continuously by the base station tofacilitate the field units demodulation of the other forward linkchannels.

[0007] The paging channel is used to inform the field unit of additionalinformation needed to communicate. Such information is typicallymanagement information which informs the field unit of which trafficchannels it may use, for example. Other types of paging messages areused to communicate system parameters, access parameters, neighbor listsand other information needed for the field unit to manage itscommunication in such a way that it does not interfere with other fieldunits transmissions.

[0008] The forward traffic channels are used to transmit user dataand/or voice signaling information from the base station to the fieldunit.

[0009] On the reverse link, there are typically at least two types oflogical channels, including an access channel and traffic channels. Theaccess channel is used by the field unit to send a message to requestaccess to traffic channels when it has data to communicate to the basestation. The field unit thus uses the access channel to make requestsfor connection originations and to respond to paging messages. Thetraffic channels on the reverse link serve the same purpose as thetraffic channels on the forward link, namely, to transmit user dataand/or digitized voice payload information.

[0010] Pilot channels are not typically used on the reverse link. Thereare perhaps several reasons for this. For example, the most widelydeployed CDMA systems, such as the IS-95 compatible system as specifiedby the Telecommunications Industry Association (TIA), use asynchronousreverse link traffic channels. It is typically thought that the overheadassociated with allowing each field unit to transmit on its owndedicated pilot channel is not necessary. It is also thought that theoverhead associated with decoding and detecting a large number of pilotchannels back at the base station would not justify any perceivedincrease in performance.

SUMMARY OF THE INVENTION

[0011] In general, pilot signals are advantageous since they provide forsynchronous communication. If the communications on the reverse linktraffic channels can be synchronized among various field units,parameters can be better optimized for each link individually. It wouldtherefore be advantageous to make pilot signals available for use on thereverse link.

[0012] Furthermore, the use of pilot channels on the reverse link wouldassist in combating effects due to multipath fading. Especially in urbanenvironments where many tall buildings and other surfaces may reflectradio signals, it is common for not just one version of each transmittedsignal to arrive at a receiver. Rather, different versions of aparticular transmitted signal, each associated with a particular delay,may be actually received. Having additional synchronization timinginformation available at the base station can help properly decodereverse link signals which have experienced a multipath fade.

[0013] The present invention is a technique for efficient implementationof pilot signals on a reverse link in a wireless communication systemencompassing a base station which services a large number of fieldunits. According to one aspect of the invention, an access channel isdefined for the reverse link such that within each frame or epoch, apreamble portion of the frame is dedicated to sending only pilotsymbols. Another portion of each access channel frame, called thepayload portion, is then reserved for sending data symbols. In thispayload portion of the frame, additional pilot symbols are interleavedamong the data symbols.

[0014] In the preferred embodiment, the pilot symbols are inserted atpredictable, regular intervals among the data symbols.

[0015] The preamble portion of the access channel frame allows forefficient acquisition of the access signal at the base station, andprovides a timing reference for separating the data and pilot symbols inthe payload portion, as well as a timing reference for, optionally,dealing with the effects of multipath fading. This is accomplished byfeeding the preamble portion to a pilot correlation filter. The pilotcorrelation filter provides a phase estimate from the pilot symbols inthe preamble portion, which is then used to decode the data symbols inthe payload portion.

[0016] An access acquisition portion of the receiver then uses thesephase estimates provided by the pilot correlation filter to process theoutput of a data symbol correlation filter.

[0017] The additional pilot symbols embedded in the payload portion arepreferably used in a cross product modulator to further undo the effectsof multipath fading.

[0018] The preamble portion of the frame may be defined by Barkersequences, which further assist with properly aligning the timing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0020]FIG. 1 is a block diagram of the system which uses embedded pilotsymbol assisted coherent demodulation according to the invention.

[0021]FIG. 2 is a detailed view of the format of data framing used onthe access channel.

[0022]FIG. 3 is a high level diagram of the pilot symbol assisteddemodulation process.

[0023]FIG. 4 is a more detailed view of the pilot symbol assistedcoherent demodulators.

[0024]FIG. 5 is a still more detailed view of an access acquisitionportion of the coherent demodulator.

[0025]FIG. 6 is a more detailed view of a data detection portion of thecoherent demodulator.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0026] Turning attention to the drawings, FIG. 1 is a generalizeddiagram showing a wireless data communication system 10 that makes useof an access channel having embedded pilot symbols in order toeffectuate coherent demodulation. The system 10 consists of a basestation 12 and a field unit 20. The base station 12 is typicallyassociated with a predetermined geographic region 14 in which wirelesscommunication service is to be provided.

[0027] The base station 12 contains several components, including aradio transmitter 15, receiver 16, and interface 17. The interface 17provides a data gateway between the base station 12 and a data network18 such as the Internet, a private network, a telephone network, orother data network.

[0028] The field unit 20 consists of a corresponding receiver 21,transmitter 22, and interface 23. The interface 23 permits the fieldunit 20 to provide data signals to and receive data signals fromcomputing equipment 24 such as a laptop computer, Personal DigitalAssistant (PDA), or other computing equipment. The interface 23 may be aPCMCIA bus, USB port, or other standard computer interface.

[0029] The base station 12 communicates with the field unit 20 byexchanging radio signals over various radio channels. The presentinvention is of particular advantage in a system 10 which uses CodeDivision Multiple Access (CDMA) modulation to define the channels. Inthe specific embodiment discussed herein, it is therefore understoodthat a specific pseudorandom (PN) code (which may or may not beaugmented with orthogonal codes) is used to define each of the variouslogical channels on a given radio carrier frequency.

[0030] The forward link 30 consists of various types of logicalchannels, including at least a pilot channel 31, a paging channel 32,and one or more traffic channels 33. The forward link 30 is responsiblefor forwarding data signals from the base station 12 towards the fieldunit 20.

[0031] The pilot channel 31 contains typically no baseband information,but rather a stream of bits that are used to permit the field unit 20 tosynchronize to the signals sent in the other forward link logicalchannels such as the paging channel 32 and traffic channel 33.

[0032] The paging channel 32 is used to transmit messages from the basestation 12 to the field unit 20 that control various aspects ofcommunication, but most importantly, control assignment of varioustraffic channels 33 for use by each field unit 20.

[0033] The forward traffic channels 33 are used to transmit data voiceor other signaling messages from the base 12 towards the field unit 20.

[0034] Signals are also carried from the field unit 20 towards the basestation 12 over a reverse link 40. The reverse link 40 contains severallogical channel types including at least an access channel 41, asynchronization (sync) channel 42, and one or more traffic channels 43.

[0035] For the reverse link 40, the access channel 41 is used by thefield unit to communication with the base station 12 during periods oftime when the field unit 20 does not have a traffic channel 43 alreadyassigned. For example, the field unit 20 typically uses the accesschannel 41 to originate request for calls as well as to respond tomessages sent to it on the paging channel 32.

[0036] The sync channel 42 on the reverse link may assist in or with thetraffic channels 43 to permit the field unit 20 to efficiently send datato the base 12 using synchronous modulation techniques.

[0037] The present invention relates to the formatting and use of thereverse link access channel 41. Specifically, the invention uses anaccess channel 41 that contains within it certain formatting such ascertain symbols used to convey pilot signal information.

[0038] The access channel 41 signal format is shown in more detail inFIG. 2. An epoch or frame 50 consists of a preamble portion 51 andpayload portion 52. The preamble 51 is further defined as a series ofsymbols including a pilot block 53 and Barker code block 54. Multiplepilot blocks 53 and Barker code blocks 54 make up the preamble 51; inthe illustrated preferred embodiment, a pilot block 53 and Barker block54 are repeated four times in each frame 50. The Barker blocks 54 assistin allowing the receiver to determine where the start of a frame 50 is.

[0039] Each pilot block 53 consists of a number of repeated pilotsymbols. In the preferred embodiment, 48 pilot symbols are repeated ineach pilot block 53. The pilot blocks 53 are used to assist with timingreception and decoding of the information symbols which make up theaccess channel 41.

[0040] The second portion of each frame 50 is the payload portion 52.The payload portion 52 includes a data portion consisting of theinformation to be sent from the field unit 20 to the base 12. As shownin FIG. 2, pilot symbols 53 are inserted in the data portion of thepayload 52. A pilot symbol, for example, may be inserted every eightpayload symbols. As will be discussed in greater detail later, thesepilot symbols embedded in the payload portion 52 further assist with thecoherent demodulation process of the information contained in the dataportion.

[0041] The pilot symbols 53 typically consist of a series of positivedata bits only. Therefore, they do not in and of themselves containtiming information.

[0042] The Barker blocks 54 may consist of predetermined patterns ofbits, as shown in FIG. 2. Binary Phase Shift Keyed (BPSK) bit encodingmay be used to indicate a Barker sequence consisting of three positivebits followed by three negative bits, followed by a single positive bit,a pair of negative bits, a positive bit, and then a negative bit. Thepositive logic Barker sequence +B may be alternately sent with thenegative of the Barker sequence −B to further assist in aligning thebeginning of each frame 50 at the receiver 16.

[0043] The use of multiple pilot blocks 53 and Barker blocks 54 permitan averaging process to be performed in the acquisition of each accesschannel 41 is described further below.

[0044]FIG. 3 is a generalized block diagram of the portion of thereceiver 16 used by the base station 12 to demodulate the reverse linkaccess channel 41. As shown, the access channel receiver consists of twofunctions including access acquisition 60 and data decoding 62. In apreferred embodiment, multiple data decoding blocks 62-1, 62-2, 62-3 maybe used as individual rake receiver portions, or receiver “fingers,”tuned to different timing delays.

[0045] In general, the preamble pilot symbols are first processed by theaccess acquisition function 60. These provide generalized timinginformation which is then fed to the data decoding function 62, alongwith the payload portion 53 containing the data symbols and embeddedpilot symbols. Each of the individual fingers 62-1, 62-2, 62-N make useof the timing information provided by the access acquisition function 60to properly decode the data in the access channel.

[0046] This receiver signal processing can now be understood morereadily by reference to FIG. 4, which is a more detailed diagram of boththe access acquisition function 60 and data decoding function 62. Inparticular, the access acquisition function 60 is seen to include aPilot Correlation Filter (PCF) 70 as well as an integration function 72.As will be discussed in more detail below, the PCF 70 is a matcheddigital filter having coefficients matched to provide an impulseresponse to input preamble pilot signals.

[0047] The integration function 72 operates on successive outputs of thepilot correlation filter 70 to provide a smoothed estimate of timinginformation inherent in the pilot symbols.

[0048] The data decoding portions 62 each include a data matched filter80, a selection function 82, a dot or “cross” product function 84,integration functions 86, and delay 88. A summer 90 operates on theoutputs of the individual data decoders 62-1 , 62-2, . . . 62-n toprovide an estimate of the payload data. Briefly, each of the datadecoders 62 operates as a synchronous demodulator to provide an estimateof the data symbols for a given respective possible multipath delay.Although three data decoders 62 are shown in FIG. 4, it should beunderstood that a smaller number of them may be used depending upon theanticipated number of multipath delays in the system 10.

[0049]FIG. 5 is a more detailed block diagram of the access acquisitionportion 60. This circuit includes the previously mentioned pilotcorrelation filter 70 in the form of a pair of pilot correlation matchedfilters (PCMFs) 700-1, 700-2, and a corresponding pair of vectorinfinite impulse response (IIR) filters 710-1 and 710-2. In addition,the integration function 72 is provided by the pair of magnitudesquaring circuits 720-1 and 720-2, a summer 722, and threshold detector724.

[0050] In operation, the access channel 41 signal is fed to the pilotcorrelation matched filter (PCMF) sections 700-1 and 700-2. The pair ofPCMFs 700 are used in a ping pong arrangement so that one of the PCMFsmay be operating on received data while the other PCMF is having itscoefficients loaded. In the preferred embodiment, the access channel isencoded using 32 PN code chips per transmitted symbol. At the receiver,8 samples are taken per chip (e.g., 8 times the chip rate of 1.2288megahertz (MHz)). The pilot correlation matched filter 700 must not onlybe matched to receive the pilot symbols, but also to the particularpseudorandom noise (PN) code used for encoding the access channel. Acontroller 730 is used to control the operation of the two portions ofthe access acquisition circuit 60, both the top half and bottom half, asillustrated.

[0051] Continuing with the discussion of the Pilot Correlation Filter70, the vector IIR filter 710-1 receives the output of the PCMF 700-1 inthe form of in-phase (I) and quadrature (Q) samples. As shown in thesignal diagram 750 next to the output of the PCMFs 700, the output tendsto be a series of peaks spaced apart in time, with the peak spacing,depending upon the multipath delays experienced on the reverse link. Forexample, a peak occurring at a first time T1 may be associated with themost direct signal path taken. A second peak may occur at a time T2associated with a portion of the signal which follows an alternate path.Finally, a third peak may be associated with a time T3 which follows yeta different path from the field unit 20 to the base 12. The series ofpeaks are output for each of the 48 symbols in the pilot burst. Thefunction of the vector IIR filter 710-1 is thus to average these pilotbursts to provide a more well defined set of peaks 760 which representsthe outputs of the PCMF 700-1 averaged over time. The averaging processimplemented by the vector IIR filter 710-1 may, for example, eliminate afalse peak, such as that occurring at time T4, which is attributable toa noise burst and not to an actual multipath signal portion.

[0052] The output 760 of the vector IIR filter 710 thus represents anestimate of where the true multipath peaks occur in the reverse linkaccess channel 41.

[0053] Of ultimate interest is the signal level of the received pilotsignal. To determine this level, the magnitude block 720-1 takes themagnitude of the vector IIR output signal 760. The sum circuit 722 thussums these signals as provided by each of the two ping pong branches700. A threshold detector 724 is then applied to the summed signal toprovide an output similar to the plot 770. The threshold detector is setat a predetermined amplitude TH so that an output appears as in plot780.

[0054] The points at which the summed signal output crosses thethreshold TH indicate points at which rake fingers 62 will be assignedto processes the signal. In particular, the peaks occurring at times T1,T2 and T3 are examined, and each respective time is used and assigned toa respective data matched filter 80 and the corresponding finger 62.These provide an estimate of possible phases from the pilot symbolswhich is in turn used in the data decoding process.

[0055]FIG. 6 illustrates how the data detection process of the threerake fingers 62. Each finger 62 is identical. An exemplary rake finger62-1 consists of a corresponding Data Correlation Matched Filter (DCMF)80-1, a peak sample detector 81-1, a switch 82-1, a vector IIR filter83-1, complex conjugate function 85-1, and dot product circuit 84-1.

[0056] In operation, the access channel signal is first fed to the DataCorrelation Matched Filter (DCMF) 80-1. This filter 80-1 is loaded withcoefficients at a specific phase delay of the PN sequence. In thisinstance, the phase delay loaded is that data associated with the timeT1 indicated from the output of the access acquisition block 60.

[0057] The output of data correlation matched filter 80-1 will consistof a signal having a localized peak. As shown in the diagram next to thepeak sample detector 81-1, the peak sample detector 81-1 selects apredetermined number of samples around this peak for further processing.

[0058] These peak values are then fed to the switch 82-1. The switch82-1, under the operation of the data decoder controller 790,alternately steers the peak detected signal, depending upon whether itcontains pilot symbols or pilot plus data symbols. The decodercontroller 790 may be synchronized with a start of frame indication asdetermined by the received Barter symbols in the preamble portion, andtherefore knows the position of pilot symbols in the payload portion.Thus, while receiving the payload or data portion 52 of the accesschannel frame 50, the signal will be steered to the lower leg 88-1, inthe case of receiving a pilot symbol, or in the case of receiving a datasymbol, will be steered to the upper leg 89-1.

[0059] The pilot symbols of the payload portion 52 are processed in amanner similar to the pilot symbol processing in the preamble portion51. That is, they are processed by a vector IIR filter 83-1 to providean averaged estimate of an estimate signal value [p]e^(j). The complexconjugate of this pilot estimate is then determined by the complexconjugate circuit 85-1.

[0060] Data symbols steered to the upper leg 89-1 provide a dataestimate signal x_(n)e^(je).

[0061] The two estimate signals, data and pilot are then fed to themultiplier 84-1 to provide a cross product of the pilot symbols with thedata symbols. This causes the phase terms of the complex signal tocancel more or less. That is, the phase estimate (theta) should beapproximately equal to the measured phase theta of the pilot symbols.The output thus represents the pilot channel energy |p|.x_(n). Given apilot symbol normalized value of 1, the data is therefore recovered.

[0062] Returning to FIG. 4, the reader will recall that this is theoutput of only one rake finger 62-1. Each rake finger output is,therefore, then fed through the integrators 86, 87, additional dotproduct circuits 89, and delays 88-1, to the summer 90 to provide afinal estimate of the data, X.

[0063] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method for processing access channel signals ina digital wireless communication system comprising the steps of: at atransmitter, encoding pilot symbols in a preamble portion of an accesschannel frame of information to be transmitted on the access channel;and encoding data symbols in a payload portion of the access channelframe, the payload portion of the access channel frame also includingpilot symbols interleaved with the data symbols.
 2. A method as in claim1 additionally comprising the steps of, at a receiver, obtaining a pilotsymbol phase estimate by feeding the pilot symbols in the preambleportion to a pilot correlation filter; obtaining a data symbol estimateby feeding the data symbols in the payload portion to a data symbolcorrelator; and using the pilot symbol phase estimate provided by thepilot correlation filter to synchronize detection of the data symbols.3. A method as in claim 2 additionally comprising the steps of, at areceiver: extracting pilot symbols from the payload portion; andperforming a cross product operation with the pilot symbols embedded inthe payload portion and the data symbols.
 4. A method as in claim 2additionally comprising the steps of, at the receiver, extracting pilotsymbols from the payload portion; and performing a cross productoperation between the pilot symbols embedded in the payload portion andthe data symbols output by the data symbol correlator.
 5. A method as inclaim 1 wherein the pilot symbols are interspersed at regular intervalsin the payload portion.
 6. A method as in claim 2 wherein thetransmitter is located at a base station, and the receiver is located atone of a plurality of field units serviced by the base station at thesame time.
 7. A method as in claim 2 additionally comprising the stepsof: detecting the pilot symbols in the preamble portion with a pilotcorrelation matched filter having a transfer function matched to thepilot symbols.
 8. A method as in claim 2 additionally comprising thesteps of: detecting the data symbols in the payload portion with a datacorrelation matched filter having a transfer characteristic matched tothe data symbols.
 9. A method as in claim 2 additionally comprising thesteps of: operating a pair of pilot correlation matched filters todetect the pilot symbols in the preamble portion, the pilot correlationmatched filters operating in ping pong such that one of the pilotcorrelation matched filters is processing a received signal while theother is loading filler coefficients.
 10. A method as in claim 2additionally comprising the steps of: receiving a payload port ionsequence of pilot symbols and data symbol s; separating the payloadportion sequence in to pilot symbols and data symbols usingsynchronization information derived from the pilot symbols in thepreamble portion; and comparing the separated pilot symbols and datasymbols to detect information received.
 11. A method as in claim 10wherein the step of comparing the separated pilot symbols and datasymbols comprises performing a dot product of the separated pilotsymbols and data symbols.
 12. A method as in claim 2 additionallycomprising the step of, at the receiver, feeding a received preambleportion to a pilot correlation matched filter; and comparing the outputof the correlation matched filter to a peak detector.
 13. A method as inclaim 12 additionally comprising the step of: determining a timeposition of a plurality of peaks in the peak detector output; andsetting a plurality of rake receivers to each of the detected peaks.